1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing an O.sub.2 sensor element that is generally used for controlling an air fuel ratio of an internal combustion engine for a vehicle.
2. Related Arts
Conventionally, O.sub.2 sensor elements of oxygen concentration electromotive force type using zirconium oxide (ZrO.sub.2) solid electrolyte and of limit current type are well-known as gas detectors for detecting an oxygen concentration in an exhaust gas from an internal combustion engine of a vehicle. These kinds of O.sub.2 sensor elements have been already commercialized.
A typical one of the O.sub.2 sensor elements is shown in FIG. 1. The O.sub.2 sensor element 9 has a cup-type solid electrolyte member 10 including an inside space 100 therein having an opening at an end thereof. The solid electrolyte member 10 further has an outside electrode 11 on an outside surface 101 thereof, and an inside electrode 94 on an inside surface 102 thereof within the inside space 100. The inside electrode 94 is composed of a reaction electrode 92 and a lead portion 93. The thus designed O.sub.2 sensor element 9 is the current mainstream because it is suitable for mass production. Further, a heater 19 is widely adopted to the O.sub.2 sensor element 9 to be held in the inside space 100 of the solid electrolyte member 10 as shown in FIG. 1, thereby obtaining sufficient operational properties at a low temperature and high flexibility of an installed position to the vehicle.
However, the O.sub.2 sensor element 9 has the following problems. That is, a temperature of the solid electrolyte member 10 around a tip portion thereof becomes high by being exposed to the exhaust gas, however, the temperature of the electrolyte member 10 is lowered as it becomes close to the opening thereof. Because of this, in a case where the reaction electrode 92 is formed on the entire inside surface of the electrolyte member 10 in which the heater 9 is no held, output properties of the reaction electrode 92 deteriorate due to the temperature distribution thereof. Further, even if the heater 19 is held within the O.sub.2 sensor element 9, there arises temperature distribution of the reaction electrode 92 due to a heating portion 190 of the heater 9, and the sensor properties are adversely affected by the temperature distribution.
Therefore, in the O.sub.2 sensor element 9 without holding the heater 19 therein, the reaction electrode 92 is disposed only around the tip portion of the electrolyte member 10. Further, in the O.sub.2 sensor element 9 holding the heater 19 therein, the reaction electrode 92 is disposed only on a specific portion of the electrolyte member 10 facing the heating portion 190 of the heater 19. Accordingly, sensor properties can be improved. In addition, in these cases, the area of the reaction electrode 92 is decreased, so that an amount of material for the reaction electrode 92 is reduced. The reaction electrode 92 usually includes noble metal such as platinum (Pt). Therefore, decrease of the area of the reaction electrode 92 means decrease of material cost.
Conventionally, as a method for manufacturing the above-mentioned O.sub.2 sensor element 9, the following method is well-known. That is, firstly, a provisionally baked or finally baked cup-type solid electrolyte member 10 having an inside space 100 is prepared. Then, a jig for coating a paste for an inside electrode is inserted into the inside space 100 of the solid electrolyte member 10. The jig has a hollow pipe formed with a plurality of holes and a porous elastic member such as polyurethane foam or the like disposed around the hollow pipe. The hollow pipe is filled with the paste. Then, the paste is extruded from the pipe to be coated on an inside surface 102 of the electrolyte member 10 through the porous elastic member within the inside space 100 of the electrolyte member 10. After coating the paste, the solid electrolyte member 10 is baked so that the paste is baked. As a result, the reaction electrode can be obtained. The above-mentioned method is disclosed, for example, in JP-A-52-94195.
JP-A-55-141665 discloses another method for manufacturing an O.sub.2 sensor element. In the method, a tapered cup-type solid electrolyte member having an inside space, which is provisionally baked or finally baked, is prepared. The inside space of the electrolyte member is filled with a paste in advance. Then, a masking jig designed to have a shape approximately the same as that of the inside space and having a concave portion corresponding to a reaction electrode that is to be formed is inserted into the inside space of the electrolyte member. In this state, a fluid pressure is applied into the inside space of the electrolyte member through the masking jig. Accordingly, the paste invades a gap between the concave portion of the masking jig and the inside surface of the electrolyte member. As a result, the paste is coated on the inside surface of the electrolyte member at the concave portion to form a reaction electrode formation portion. Finally, the solid electrolyte member is baked so that the paste is baked, and thereby the reaction electrode can be formed.
However, the above-mentioned methods have the following problems. Firstly, in both methods, as shown in FIG. 1, the reaction electrode 92 is formed not only on the portion facing the heating portion 190 but on a bottom face 103 of the electrolyte member 10 as well. The bottom face 103 does not face the heating portion 190, so that the bottom face 103 is unlikely to be sufficiently heated. Further, it is difficult that a reference gas such as air circulates around the bottom face 103. Therefore, the reaction electrode 92 needs not be formed on the bottom face 103 of the electrolyte member 10. In addition, forming the reaction electrode 92 on the bottom face 103 increases material cost for that.
Next, when the electrolyte member 10 is provisionally or finally baked, a baking temperature is liable to have variations, resulting in variations in size of the electrolyte member 10. In such a case, in the former method, when the above-mentioned polyurethane foam is inserted into the inside space 100 of the electrolyte member 10 to supply the paste therefrom, a pressure applied to the polyurethane foam and the like is changed, so that an amount of the paste supplied from the polyurethane foam has variations. This makes difficult that the reaction electrode 92 has a uniform thickness. In the later method in which the masking jig is used, the gap between the masking jig and the inside surface 102 of the electrolyte member 10 becomes non-uniform. This also makes difficult that the reaction electrode 92 has a uniform thickness. In addition, there arises a problem concerning workability for inserting or removing the jig into or from the electrolyte member 10.
Further, the above-mentioned polyurethane foam serving as the coating jig is designed to contact the inside surface 102 of the electrolyte member 10 when coating the paste. Because of this, unless the electrolyte member 10 is tapered so that the diameter thereof becomes large as it becomes close to the opening from the bottom face 103 thereof, the paste is attached to the entire area of the inside surface 102 of the electrolyte member 10 when the polyurethane foam is taken out from the electrolyte member 10. This kind of problem occurs likewise in the case where the masking jig is used. Therefore, in the both methods, the shape of the electrolyte member 10 is restricted, and thereby flexibility of design of the O.sub.2 sensor element is restricted. Further, the about-mentioned polyurethane foam is susceptible to clogging. The clogging of the polyurethane foam also causes variations in thickness of the reaction electrode 92. Furthermore, there is a problem that an output lead portion need to be formed by another process. In a case where the inside electrode 92 is formed by a chemical plating method utilizing one of the about-mentioned methods, the same problems as mentioned above can occur.